Definitive? … the 1580 copy of John Baret's Alvearie or Quadruple Dictionarie that could just have belonged to Shakespeare

If ever there was a claim which I would love to see be proved true, it is this one. In time for Shakespeare's 450th birthday – marked on Wednesday with a wealth of quizzes and "10 things you didn't know about"s and idiotic "Shakespeare would have liked Twitter" press releases – rare booksellers in New York have announced that they believe they have purchased the playwright's own dictionary.

Before we go into the whys and wherefores of it all: just imagine if it turns out to be true. Shakespeare's dictionary! It doesn't seem possible; it makes him seem all too human.

Anyway, here are the details. The booksellers, George Koppelman and Daniel Wechsler, bid on eBay in 2008 for a copy of John Baret's Alvearie or Quadruple Dictionarie, printed in London in 1580. Winning their bid, they examined the book closely, and came to believe – over the course of a six-year study – that "in the course of his intellectual development, Shakespeare did turn to Baret, not once or twice, but 'many a time and oft', and that the copy of the Alvearie they had acquired was, indeed, his own."

Koppelman and Wechsler have just published a book about their findings, Shakespeare's Beehive, and have also just launched a Shakespeare's Beehive website, which includes high-resolution facsimiles of every page from the 1580 edition of Baret's book, showing all the annotations, as well as their insights into what they believe were Shakespeare's methods.

"Over the course of discovery, it became impossible for us to neglect a host of personal markers that run throughout the annotated book – fingerprints left by the annotator that reveal a personality and hint at an identity," they say, citing "verbal parallels and echoes" throughout their edition of the dictionary, and "idiosyncratic renderings by the annotator of the letters 'W' and 'S'".

"In addition, there is a preponderance of natural history annotations, interest in the language of clothing and costume of the period critical to stagecraft, and annotations that connect with our understanding of Shakespeare's father and his profession," they say.

The website is hugely thorough, and the dictionary itself is rather stunning.Experts are just getting started on their analysis; so far the Folger Shakespeare Library isn't convinced, and has laid out a series of proofs which need to be met.

"At this point, we as individual scholars feel that it is premature to join Koppelman and Wechsler in what they have described as their 'leap of faith'," Folger experts say. But they, like the booksellers, point to TW Baldwin's earlier assertion that Shakespeare would have used Baret.

"Baret was in effect the standard English dictionary of Shakespeare's schooldays, and must have had powerful influence in shaping the English definitions of Shakespeare's generation," wrote Baldwin. "But it is not likely that Shakespeare would have preserved the patterns so accurately if he had not himself turned many a time and oft to Baret for his varied synonyms."

Michael Witmore and Heather Wolfe at the Folger say that: "Even the most sceptical scholar would be thrilled to find a new piece of documentary evidence about William Shakespeare. Scholars, however, will only support the identification of Shakespeare as annotator if they feel it would be unreasonable to doubt that identification. This is a fairly high evidentiary standard, since it requires one to treat sceptically the idea that this handwriting is Shakespeare's and to seek out counterexamples that might prove it false."

Shakespeare biographer Stephen Greenblatt was enthusiastic about the finding to the Sydney Morning Herald, telling the paper: "It would reinforce, in a fascinating way, Shakespeare's passion for language. We know that Shakespeare had an eye out for unusual words – but we have only limited knowledge of where he went to find them." He hadn't had time to weigh the evidence yet, however.

If you're as intrigued – and hopeful – as I am, there's a huge New Yorker piece to read on the whole affair here, and of course there's the (free to register) Shakespeare's Beehive website. I'm off for a proper dig around, fingers meanwhile crossed that the booksellers will be proved right. If so, the Sydney Morning Herald speculates it could be worth $100m. Moneybags to dream of indeed.